Gyratory compactors, including the SUPERPAVE Gyratory Compactors (SGC) developed in response to the Federal Highway Administration's SHRP research program, have proven to be a useful tool in the design and quality assurance programs for Hot Mix Asphalt (HMA) products. The SGC applies a constant consolidation pressure onto a material specimen, such as HMA, contained within a cylindrical mold of specified dimensions. The specimen is confined within the mold typically between two end plates which fit within the mold. While the consolidation pressure is applied, the cylindrical mold is tilted on its axis (i.e., one end of the axis displaced while the other end is held fixed) to a precise angle, known in the art as the angle of gyration, or gyration angle. The tilted axis is then gyrated around an axis perpendicular to the mold end plates, which may also be the axis along which the consolidation pressure is applied, i.e., the axis of consolidation or compaction. The gyration of the mold is performed at a specified rate.
Studies have shown that the angle of gyration, as determined by the degree of tilt of the mold axis from the axis perpendicular to the mold end plates, plays a key role in determining the amount of compaction a material specimen, such as HMA, experiences during gyration. The primary method used to measure the angle of gyration is to measure the angle between an external surface of the mold cylinder with reference to a stationary external reference, such as a member of the surrounding frame of the gyratory compactor. However, such measurement of an external angle of gyration, in addition to being subject to variances introduced by deflection of the machine frame, does not account for movement of the mold end plates within the mold resulting from the reaction forces of the material specimen in the mold (including shear forces between the material and the mold wall) which are transferred to the machine frame. It is the internal angle between the mold cylinder and the compaction/planar surfaces of the mold end plates which contact the specimen that determines the amount of compaction effort imparted to the specimen. This is referred to as the “internal angle of gyration” or “internal gyration angle”; the angle formed between the internal mold wall and the mold end plates during compaction. When two mold end plates are used in the mold, one at each end of the material specimen, the internal gyration angle can be measured with respect to each end plate. With the mold vertically oriented in an SGC and a mold end plate at the top and bottom of the specimen, these internal angle measurements are generally referred to as the top (internal) angle and the bottom (internal) angle.
U.S. Pat. No. 6,477,783 (the '783 patent) describes a device that fits inside the SGC mold along with a material specimen, such as an HMA, which is placed on top of the device while the SGC is operated. A flat end of the device is placed flush against the bottom mold plate, and an HMA specimen is placed on top of the device. Probes extend out of a side wall of the device to contact the inside wall of the mold. The device disclosed in the '783 patent is operable only with an HMA material specimen in the mold on top of the device, in order to apply the material-induced loads to the SGC frame so as to measure the internal angle of gyration under what is assumed to be realistic, material-based conditions. However, the molds used in many existing SGC models do not have sufficient volume to hold a typical size material specimen and the internal angle measuring device described by the '783 patent. The forces acting on the gyratory compactor frame are related to the material stiffness as well as to the amount of material being compacted. The inability to use typical sized specimens with the device of the '783 patent in some SGC models (or the use of undersized specimens) necessitates use of a complex procedure to accurately predict the angle with a full sized specimen. To overcome this shortcoming, it is necessary to take multiple measurements (e.g., as many as seven or more) with specimens of different masses compacted with the angle measurement device of the '783 patent, and then perform an extrapolation calculation to predict what the internal angle of gyration is with a full size specimen. Because the mold must cleaned, pre-heated and loaded with the device, and the material must be pre-heated and loaded into the mold on top of the device, the repetition of this process seven or more times can consume nearly an entire work day just to measure the internal gyration angle of a single gyratory compactor.
HMA material specimens have varying stiffness properties that can be difficult to characterize accurately, creating a situation of an unknown force being applied while the measurement is made. Each SGC model has a unique frame stiffness resulting in a characteristic different frame deflection during operation. Frame deflection may cause different angles of gyration for HMA specimens of different stiffnesses in SGC models with relatively more flexible frames. It is important to have known forces similar to that of the material typically compacted to overcome this problem. Furthermore, because the internal gyration angle measurement device of the '783 patent is used exclusively with a material sample in the mold, the compaction ram and its support structure is not in a position similar to that of normal operation, creating possible changes in frame stiffness characteristics due to the extension of the ram, which further alters the test results.
As noted above, the angle of gyration must be known with great accuracy. To make an accurate measurement, the apparatus, molds, mold end plates, and the SGC apparatus must be very clean. The device and method described in the '783 patent requires the use of HMA consisting of aggregates of various sizes mixed with asphalt binder, a tar-like substance, which makes debris a significant factor in the accuracy of this angle measurement. Also, HMA specimens are typically compacted at 150° C., making the procedure difficult and creating operator safety concerns as well as problems associated with overheating the measurement device. The extreme care necessary to minimize the effect of temperature on the measurements requires a cooling period for the instrument between measurements, adding still more time to the lengthy angle measurement, extrapolation and calibration process. The horizontal probes of the '783 patent device are aligned vertically to interface with the curved inner surface of the mold wall while the flat base contacts a mold endplate. Dirt or debris between the mold end plates and the '783 patent device can significantly effect the accuracy of the measurement. Because of the need for extreme accuracy, close attention must be used to minimize the effect of dirt or debris in the measurement.
It is therefore desirable to obtain a very accurate measurement of this internal angle of gyration under actual material loads. Even more preferable would be to obtain highly accurate measurement of the internal angle of gyration under conditions similar to those occurring during compaction of the sample material, without having material actually present in the mold. This would be particularly advantageous for angle calibration of gyratory compactors for use with HMA, to avoid pre-heating of the mold and material, and repeated loading and unloading of the mold with HMA to obtain an accurate measurement of the internal gyration angle.